The present invention relates generally to digital logic, and more specifically to a technique for communicating external information to a logic circuit on a semiconductor chip.
As semiconductor products have grown more sophisticated, the need for multi-purpose devices has increased. Thus, a large chip, such as a sophisticated controller circuit, may be required to operate in a number of modes, depending on the configuration of the system within which it is embedded. This is typically implemented by providing circuitry on the chip, responsive to one or more mode select signals or signal levels from outside the chip, for reconfiguring the chip circuitry or otherwise defining its operation. Thus, the chip might be provided with a number of dedicated pins that are tied high or low (with off-chip pull-up or pull-down resistors), as required for a particular configuration.
Fortunately, chip designers are now able to fit an incredibly large amount of sophisticated logic on a semiconductor chip, so it is possible to implement such multi-mode operation. Unfortunately, the limiting factor may have shifted from the number of active devices that can be put on the chip to the number of input or output pins that are available on relatively standard and economical packaging. In concrete terms, there are commercially available 160-pin packages costing a few dollars. Were a device to require 161 pins, a much more expensive package, perhaps a custom design, would be called for. Thus, a pin saved may well translate into many pennies earned.